Before you begin freeze the chocolate bars for at least 15 minutes and do not remove them until it is time to chop them up.
Heat the butter or margarine in a light-colored pan over low heat until deep amber with toasted flecks. Take it off the heat right away. Browned, not burnt.
Stir the cocoa, espresso powder, and hot chocolate powder into the hot fat.
In a large bowl, whisk the eggs, extra yolk, brown sugar, and vanilla until thick and combined.
Stir in the cocoa mixture. Fold in the flour and kosher salt just until combined.
Fold in the milk chocolate and white chocolate.
Pour into a square pan and bake at 350°F for 22 minutes. Don't overbake.
Sprinkle with flaky salt right after baking. Cut warm for gooey centers, or chill first for clean squares.
